This invention relates to selective catalytic reduction of NOx present in flue gas from combustion, for example coal combustion in power-generation plants. More particularly, it relates to selective catalytic reduction of NOx at low temperatures.
Selective Catalytic Reduction, SCR, technology is used worldwide to control NOx emissions from combustion sources at higher temperatures (550-750° F.). SCR/direct destruction of NOx and catalytic oxidation of Hg0 at low temperature (below 350° F.) is a relatively new field awaiting major breakthroughs to reach commercial viability.
High temperature SCR technology has been used in Japan for NOx control from utility boilers since the late 1970's, in Germany since the late 1980's, and in the US since the late 1990's. The function of the SCR system is to react NOx with ammonia (NH3) and oxygen to form molecular nitrogen and water. Industrial scale SCR's have been designed to operate principally in the temperature range of 500° F. to 900° F. but most often in the range of 550° F. to 750° F. Catalysts used in this application are sulfation-resistant metals such as vanadium, titanium and tungsten and a variety of their oxides. As used herein, a sulfation-resistant metal is one that resists reaction (or that does not readily react) with sulfur-containing species such as sulfates and sulfur-based acidic gases to form metal-sulfur (sulfate or sulfite) salts. Such sulfation-resistant metals and select oxides can support redox reactions while still being resistant to forming sulfur-based salts. These catalysts are generally preferred because they exhibit good resistance to sulfur poisoning. Other researchers in this field (Teng et al., 2001; Long et al., 2002; Chen et al., 2000; Moreno-Tost et al., 2004) have identified several metals and their oxides that showed catalytic qualities for SCR, including chromium, manganese, iron, cobalt, nickel, copper, and zinc. While these catalysts are effective for SCR-NOx reduction at lower temperatures (e.g. 350° F. or lower), they are also subject to sulfur poisoning; i.e. they are not sulfation-resistant. Such metals (and their oxides) that are not sulfation-resistant are referred to herein as common base metals. Conversely, even though conventional SCR catalysts are resistant to sulfur poisoning, they are generally ineffective at lower temperatures due to their low reactivity at the low temperature ranges (260-350° F.).
The application of base metal compounds for combined SOx and NOx control has been studied and to a limited extent practiced for three decades, although the focus has been on SOx removal. These systems operated generally in the same temperature regime as the conventional SCR (550° F. to 750° F.), and utilized means to regenerate the metal compounds after they reacted to remove SOx from the flue gas, to produce various sulfur products that could be separately removed or disposed of, such as sulfur, sulfuric acid, and ammonium sulfate. As noted above, the focus of these processes was SO2 capture, with NOx capture a secondary effect. By contrast, the present invention aims particularly to reduce or destroy NOx in flue gas, as well as mercury oxidation. Sulfur capture to the extent that it happens in the novel SCR constructions disclosed herein, within the prevailing low temperature range, would be considered an interferent in the following description.
Very little is known on the direct catalytic destruction of NOx in the absence of NH3. Yokomichi et al. (2000) presented results on direct NOx decomposition by copper-exchanged zeolites at high temperatures (570-1110° F.). The presence of oxygen in the flue gas and lowering the catalyst temperature had a negative impact on the activity of these catalysts.
Catalytic oxidation of Hg0 to its oxidized forms (Hg2+) is of interest due to the solubility and ease of control of Hg2+ in wet scrubbers. Ghorishi (1998) studied the effect of several common metal oxide catalysts on Hg0 oxidation. That study suggested that cupric oxide (CuO) and ferric oxide (Fe2O3) are very active in promoting the oxidation of Hg0 in the presence of hydrogen chloride (HCl) in the flue gas. CuO exhibited a much higher activity in that work. The Hg0 oxidation activities of these two metals were hypothesized to be caused by the Deacon process in which chlorine gas (Cl2) is catalytically produced from HCl over these two oxides. Hg0 was then oxidized by reacting with Cl2 in the vicinity of the surface of the catalyst. In a follow up study, Ghorishi (1999) showed that cuprous chloride (CuCl) has a far superior catalytic activity than CuO. It was found that CuCl was so reactive that it caused the oxidation of Hg0 even in the absence of HCl in the flue gas. In a later study, Ghorishi et al (2005) used a two-step global Deacon reaction scheme (Nieken and Watzenberger, 1999) to explain the superior activity of CuCl. This two-step mechanism divides the Deacon process into a chlorination step (which results in the formation of an intermediate surface species, CuCl2 or perhaps CuCl, and the release of gas-phase H2O) and a dechlorination step (which results in the formation of Cl2 and the regeneration of the original CuO catalysts):Chlorination: 2HCl+CuO→CuCl2+H2ODechlorination: 2CuCl2+O2→2CuO+2Cl2 Net Deacon process: 4HCl+O2→2Cl2+2H2O
Ghorishi et al. (2005) hypothesized that by using a copper chloride catalyst the chlorination step and thus the presence of HCl in flue gas would no longer be needed. Elimination of the chlorination step would also lead to a faster Hg0 oxidation reaction and thus the superior activity of the CuCl catalyst. It should be noted that in the case of CuCl2/CuCl catalyst and the absence of HCl in the flue gas, there would be no regeneration and the catalyst would be eventually exhausted to CuO. At that time, the chlorination step would become important and the presence of HCl may be needed to regenerate the copper chloride catalyst material according to the Chlorination reaction shown above.
SO2 poisoning of CuCl and/or CuCl2 is also major concern regarding activity of Hg0 oxidation catalyst. As noted above in the context of NOx reduction, some transition metal compounds such as CuCl and CuCl2 are susceptible to sulfur poisoning, which can result in production of the metal sulfate and depletion of the useful catalyst material (metal halide).
Accordingly, an apparatus and method for the continuous regeneration of the these-metal oxide catalysts (or the continuous introduction of fresh catalyst) is desirable to reverse or minimize the effects of sulfur poisoning. Such a system would enable sulfation prone metal or metal oxide catalysts to be used in low-temperature NOx-reduction and Hg-oxidation reactors, such as in a low-temperature SCR operating in an electric power generation plant. Operating the SCR at low temperature would open up a broad range of boiler-installation designs (including SCR-retrofit locations) that would no longer require the SCR to be located upstream of the air heater (which preheats combustion air entering the boiler via heat exchange with exiting flue gas) relative to the flue-gas flow path.